Mid-voltage (e.g. 30V-300V) trench type power MOSFETs that include insulated gate electrodes and an insulated source field electrode in a common trench are known. One problem of extending the mid-voltage design to lower drain voltages is the difficulty in forming the gate electrode adjacent to the source field electrode in the trench. Another problem is the difficulty in forming and then later etching the thinner oxide that insulates the source field electrode.
A method according to the present invention overcomes the difficulties outlined above by forming the common trench in a two-step trench etch process.
Specifically, a process according to the present invention includes forming a first trench mask, that may include a silicon nitride layer, over one surface of a semiconductor body, the first trench mask having an opening therein; removing semiconductor material from the semiconductor body from a bottom of the opening in the mask to obtain a first trench having a first width and extending to a first depth inside the semiconductor body, the first trench including sidewalls and a bottom; forming spacers inside the first trench, each spacer extending from the bottom of the first trench along a respective sidewall thereof, the spacers being spaced from one another at the bottom of the first trench; removing semiconductor from the bottom of the first trench using the spacers as a second trench mask to obtain a second trench having a second width and extending to a second depth below the first depth inside the semiconductor body, the second width being less than the first width, the second trench including sidewalls and a bottom; covering the sidewalls and the bottom of the second trench through, for example, oxidation of the same or deposition of a suitable insulation (e.g. deposition of silicon dioxide or some other suitable insulation body); and forming a field electrode inside the second trench, the source field electrode extending into the first trench.
In one embodiment, the spacers are comprised of silicon dioxide, which are then removed to allow for formation of insulated gate electrodes adjacent the source field electrode.
In another embodiment, the spacers are insulated gate electrodes.
A device fabricated according to the present invention can include a second trench having vertically oriented sidewalls or sidewalls that taper toward the bottom thereof.
In a process according to the present invention, the width and the depth of the first trench can be selected to optimize the spacer oxide etch, the gate electrode width and the desired figure of merits (FOMs). FOMs that are at least partially controlled by the gate depth and width are Rdson, gate charge, and breakdown voltage.
A method according to the present invention advantageously:                1) increases the process window for creating the gate electrode;        2) enables the formation of gate electrodes whose width can be easily controlled and defined by the trench etch process;        3) eliminates the need for multiple sacrificial oxide steps that will be required to form the gate electrodes; and        4) allows for better control of the Qgd, Qgs.        
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.